Moved content container interfaces into their own files

[freemanjp]

To make these interfaces easier to find.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #39 into master will not change coverage.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master      #39   +/-   ##
=========================================
  Coverage     99.74%   99.74%           
- Complexity       42      106   +64     
=========================================
  Files            44       70   +26     
  Lines           393      393           
  Branches         33       33           
=========================================
  Hits            392      392           
  Misses            1        1

Impacted Files	Coverage Δ	Complexity Δ
...tsign/maven/doxia/sink/kotlin/content/Paragraph.kt	100% <ø> (ø)	1 <0> (ø)	⬇️
...n/maven/doxia/sink/kotlin/content/TextContainer.kt	100% <ø> (ø)	5 <0> (?)
...en/doxia/sink/kotlin/content/PageBreakContainer.kt	100% <ø> (ø)	1 <0> (?)
...n/maven/doxia/sink/kotlin/content/UnorderedList.kt	100% <ø> (ø)	1 <0> (ø)	⬇️
...xia/sink/kotlin/content/HorizontalRuleContainer.kt	100% <ø> (ø)	2 <0> (?)
...aven/doxia/sink/kotlin/content/NumberedListItem.kt	100% <ø> (ø)	1 <0> (ø)	⬇️
...en/doxia/sink/kotlin/content/DefinitionListItem.kt	100% <ø> (ø)	1 <0> (ø)	⬇️
...tsign/maven/doxia/sink/kotlin/content/TableCell.kt	100% <ø> (ø)	1 <0> (ø)	⬇️
.../maven/doxia/sink/kotlin/content/DefinitionList.kt	100% <ø> (ø)	1 <0> (ø)	⬇️
...b/gantsign/maven/doxia/sink/kotlin/content/Date.kt	100% <ø> (ø)	1 <0> (ø)	⬇️
... and 76 more

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